Field of the Invention
Aspects of the present disclosure generally relate to image forming and, more particularly, to an image forming apparatus capable of performing color correction and a color correction method for the image forming apparatus.
Description of the Related Art
Image forming apparatuses allow calibration for the purpose of preventing or reducing variations occurring due to various factors and of stabilizing the color of an image to be formed by the image forming apparatus. There have been conventionally proposed many techniques about the calibration, which is a color correction method for an image forming apparatus. The proposed techniques include a calibration specialized in gray correction, which is called “G7 calibration”, as discussed in U.S. Patent Application Publication No. 2008/0273052 A1.
FIG. 3 illustrates a gray determination patch, which is usable for the G7 calibration.
FIG. 6 illustrates gray correction for each density region in the G7 calibration.
The G7 calibration is a technique to generate a correction curve used to correct the single color of each of cyan, magenta, yellow, and black (CMYK).
The G7 calibration executes the following processes (1) to (3):
(1) After generating correction curves used to correct CMYK single-color gradation characteristics based on the density values, fixing the signal value (the amount of toner to be used) of cyan with respect to a gray formed with CMY toners (hereinafter referred to as a “CMY mixed-color gray”);(2) Then, performing colorimetry on a gray determination patch 301, which is composed of a plurality of segment patches that have consecutive signal values obtained by changing the signal value of magenta and the signal value of yellow toward positive values and negative values; and(3) From among the plurality of segment patches, which constitutes the gray determination patch 301, determining adjustment values, which are a density value of magenta and a density value of yellow, that are set in a patch closest to gray in chromaticity value.
The G7 calibration prints the gray determination patch 301 in a plurality of density regions extending from a low density region to a high density region (the sum of C, M, and Y signal values of which is 300%). Then, the G7 calibration performs colorimetry on the gray determination patch 301 and determines adjustment values for magenta and yellow used for gray correction, thus generating respective correction curves for C, M, Y, and K. The G7 calibration reflects the generated correction curves in the density values of CMYK to be output from the image forming apparatus, thus enabling the color reproduction of a gray that is more visually neutral.
However, the above-mentioned G7 calibration is a technique premised on the assumption that an image forming apparatus used for printing is of the offset printing system. The offset printing apparatus is capable of reproducing the color of a gray that is neutral to some degree even if the sum of CMY signal values is 300%.
On the other hand, with regard to an image forming apparatus of the dry electrophotographic system, although depending on the characteristics of the apparatus, when the sum of CMY signal values becomes more than 200%, toner may become unable to be completely transferred or fixed to an image forming medium.
Therefore, an image forming apparatus of the dry electrophotographic system is generally provided with an application amount limiting device for a color material usable for one fixing operation. If the amount of the color material usable for one fixing operation exceeds an application amount limiting value, the application amount limiting device performs processing for changing the combination of signal values corresponding to the amount of the color material to change the signal values in such a manner as to prevent the amount of the color material from exceeding the application amount limiting value.
Under the influence of the phenomenon of transfer and fixing failure and the change of signal values, the absolute a* and b* values in the L*a*b* color space of the CMY mixed-color gray often deviate from the gray axis in a high density region.
As the absolute a* and b* values become great, in other words, deviate from the gray axis, in the L*a*b* color space, the gray may visually appear as a gray mixed with some color such as green or red.
FIG. 8 is a graph representing the deviation from the gray axis in image forming apparatuses.
A dashed line and a two-dot chain line in the graph of FIG. 8 each indicate the deviation from the gray axis in the GRACoL (General Requirements for Applications in Commercial Offset Lithography) standard based on image forming apparatuses of the offset system in the United States. On the other hand, a solid line indicates the deviation from the gray axis in image forming apparatuses of the dry electrophotographic system.
Furthermore, the abscissa axis of the graph indicates the sum of signal values of the CMY mixed-color gray. The ordinate axis indicates the deviation from the gray axis of the CMY mixed-color gray. The values indicated on the ordinate axis are expressed with C* values (chroma values) in the CIE L*C*h* color space.
Referring to the graph of FIG. 8, it is understood that, with regard to image forming apparatuses of the dry electrophotographic system, the CMY mixed-color gray greatly deviates from the gray axis, in other words, the chroma value becomes great, in high density regions. A double line depicted in parallel with the abscissa axis indicates a predetermined threshold value Th. In the example illustrated in FIG. 8, the threshold value Th is set to “1.5” in delta C*.
FIG. 7 is a graph indicating correction curves for CMY in image forming apparatuses of the offset system.
FIG. 9 is a graph indicating correction curves for CMY in image forming apparatuses of the dry electrophotographic system.
When the gray correction is performed with the deviation from the gray axis set for image forming apparatuses of the offset system illustrated in FIG. 8, the correction curves are smoothly curved as illustrated in FIG. 7. On the other hand, when gray correction is performed with the deviation from the gray axis set for image forming apparatuses of the dry electrophotographic system (the deviation from the gray axis is great in high density regions) illustrated in FIG. 8, the correction curve for magenta is curved in a complicated shape as illustrated in FIG. 9, so that gradation steps occur.
Accordingly, if the gray correction such as the G7 calibration is performed in image forming apparatuses of the dry electrophotographic system, the color reproduction of neutral gray becomes available. However, on the other hand, since gradation steps occur in single colors of CMY, the gradation characteristics of single colors of CMY may be greatly impaired.